Nomenclature

The series name MD-4P is cited by Korean website ePartsHub, with a picture of Sandy's bar mount switch and the description "various switches".[1] With MD-4PCS being a known part number, this would put both bar and cruciform mount in the same series. However, the cruciform mount, illuminated version is cited by a related website as "MD-7", suggesting MD-4P is the bar mount version, MD-4PCS is the cruciform mount version, and that "MD" is the common aspect of the part numbers.[2] Provisionally this is being used as the series name.

ePartsHub cite the switch as being made by Sejin. The only known PCB photograph of the Futaba Texas Instruments TI-99/4A keyboard bears both the Futaba and Sejin logos, but all cited instances of the product family (Futaba MD-4PCS, the Acorn BBC Microcomputer simplified linear switches, and the complicated linear switches in the Lear Siegler ADM 36) all cite simply Futaba as the source. While Sejin may have been the manufacturer, the series was clearly marketed by Futaba.

The previous "complicated" moniker arose due to the significantly greater complexity over the visually very similar Futaba simplified linear.

Description

Like most other Futaba linear switches, the shell is cylindrical with truncated sides, with a square top above the plate. It sits in a circular hole in the plate. The legs are threaded at the top and screw into place; the base plug of the shell is retained in place by the legs. The legs are forced inside small holes in the contact plates, and in forcing the holes larger, the legs make a good connection with the contact plates.

The switch has been seen in black with a deep orange base plug, in white with an orange base plug, all black, and all white. The black version with the orange base cap has part number MD-4PCS.[3]

The switch comprises 16 parts in total, making it one of the most intricate switches ever designed (more so as it is only a linear switch):

Shell

Base plug

Two threaded legs

Return spring

Leaf spring

Contact assembly:

Base

Lower and upper contact leaves

Plastic separator

Rubber top

Seal

Slider:

Main slider shaft

Separate slider side (drops into the main shaft)

Rubber ring

Black dot

Operation

Two contact plates are placed around a plastic separator, inside a flat base. Above this is a thin flexible rubber sheet, which is sealed onto the sides of the base via an adhesive square. The leaf spring runs parallel to the slider, and it has a central tine with a nub that presses down on the rubber sheet. Via the rubber sheet, the pressure is transferred to the flexible legs of the movable contact, which touch the stationary contact.

The contacts have long exterior legs, with holes in them. The legs of the switch are of a greater diameter than these holes; when first screwed into the switch, they force a larger hole into the contact plates. As the metal is twisted inwards, it makes lateral contact against the sides of the legs.

GRI KBM

This switch was also sold by GRI as the KBM (keyboard switch, mechanical).[4] While GRI keyboards and keypads have been found with the standard "bumblebee" colour scheme, GRI also had bright white switches made with GRI branding.[5]

KBM switches are described as having 3.5±0.8 oz operating force, which at just short of 100 g would make this the total force; assuming 35 g preload, then the actuation force would be around 67 g. Pretravel is given as 0.03±0.01″ and total travel as 0.12±0.01″; this gives pretravel of 0.8 mm and total travel of 3 mm.

GRI's part numbers are:

KBM-01-01

Momentary

KBM-01-01-AA

Alternate action

Specification

The following table compares the specification provided by GRI with that reported to be found in FS-5104P4.[1]

Variants

Cross mount

The cross mount version has a non-uniform cruciform slider approximately 4.0 mm by 4.7 mm, with 1.2 mm wide arms, taking a 4 mm deep keycap stem. It has been seen in black with an orange base cap, white with an orange base cap, all black[6], and all white.

The white/black/orange version has part number MD-4PCS. GRI KBM switches are also this variant.

Bar mount

All-black shell with a white slider[7], white[8] (base plug not visible), and black with an orange base and white slider.[9] One minor difference between the examples examined, is that the bar mount variant was found to have a transparent plastic cover over the contact assembly, instead of black rubber-like material.

Multiple-switch return key

TeleVideo keyboards (specifically TS-800A, TS-802 and TS-825) have been found with dual switches for the Return key. [10] These switches are white with a cream base plug, and otherwise appear to be identical to the white/orange switches used on the rest of the keyboard. The reason for having two switches side by side instead of a stabiliser is not known, nor why the switches are a different colour.

Illuminated

The illuminated version, with a crucform keystem, is given as MD-7;[2] it is not clear whether this is a series name or model number.